This proposal is directed at those surface membrane changes which occur when: 1. cultured mouse embryo fibroblasts enter a quiescent state designated as "GO", and 2. following stimulation of these cells, and cell-derived surface membrane vesicles, by purified mitogens such as fibroblast - and epidermal growth factor (FGF and EGF, respectively). Those systems to be examined include normal, transformed, and transformtion - revertant cells capable of being GO - or Gl - arrested. Membrane functions to be analyzed during entrance into, and exit from quiescence will be those involved in transport and transport-related enzyme actvity. specifically, this will include uptake of A-group amino acids and free uridine, as well as Na ion/K ion-ATPase activity. The following objectives are to be pursued: 1. a determination of those membrane-function changes in quiescent cells which are GO-specific; 2. a determination of those changes which occur immediately after treatment of quiescent cells with a purified mitogen(s); 3. use of an in vitro plasma membrane vesicle system to study membrane-fuction alterations involved in growth regulation. The methodology will include induction of the quiescent state by serum growth factor-deprivation. Transport and membrane enzyme-activity determinations will be performed using radioisotopically-labeled substrates, and the biological systems used will include both intact cells and surface membrane vesicles. Overall, data will be obtained on surface membrane changes related to quiescence and GO, and on the response of cells and cell-derived membrane vesicles to purified mitogens such as FGF and EGF. This data will be used to generate a model pertaining to the mechanism by which some mitogens interact with the cell's surface membrane during the initial phase of growth stimulation.